Location of radio transmitters is a technology that has been used for many years and in many applications. For radio location, the use of mobile transmitters and a plurality of fixed base station receivers is generally known in the art where the fixed receivers locate the source of the radio signal by triangulation of the radio signals. Triangulation is a technique where a plurality of fixed base station receivers each determine the approximate direction of the radio signal from the transmitter. The approximate direction lines are then drawn on a map and the location of the transmitter is defined at the cross point of these lines. Since the direction lines are approximate, and typically three fixed base station receivers are used, the intersecting area between the three lines is usually shaped like a triangle since the lines rarely meet at a single point. The transmitter is then located within the triangle.
Triangulation of radio signals is a costly undertaking since at least two receiving antenna stations must be situated at different diverse geographic locations to perform even rudimentary triangulation. Effective triangulation is more realistically achieved with three receiving locations and accuracy is increased even more with four receiving towers. The more receiving stations constructed, the better the accuracy in locating the transmitter, but also the great the cost.
One such direction finding technique is described in one of the patent applications of the present invention, U.S. patent application Ser. No. 08/329,523 entitled "MULTIPATH-RESISTANT FREQUENCY-HOPPED SPREAD SPECTRUM MOBILE LOCATION SYSTEM" filed Oct. 26, 1994, which is not prior art to the present invention. In that patent application, the direction of the incoming radio signal is determined by using an array of receiving antennas to measure the phase difference between the antennas of the array and thereby calculate the incident direction of the radio carrier signal. Although the parent patent application has many novel and useful advantages, it requires that each base station use an array of antennas and at least two and typically three bases. Quite often the cost of building and maintaining these base stations can be quite high, especially if they are located in urban areas where roof-top space must be leased and the cost of the lease is determined by the number of antennas placed thereon.
Another disadvantage of triangulation for location of radio transmitters is the construction of such a system in sparsely-populated areas. In the example of a highway across a desert area, it is cost prohibitive to install a large number of base stations to track a narrow stretch of road. The base stations would have to be placed on both sides of the road and spaced quite far back from the road.
There is a need in the art to locate a radio transmitter using at least three and in special cases four one-antenna receivers. A receiver with one antenna will minimize the construction and lease costs of a location system. There is a further need in the art to determine the location of low power, frequency hopped spread spectrum radio transmitter.